This invention relates to surge protectors, and in particular to balanced dual-gap protectors.
In a variety of apparatus, protection against surges due to lightning, induced AC power, power faults, and other causes is essential. A standard type of protector, used for example in telephone systems, is the sealed gas surge limiter. This device typically includes a pair of electrodes mounted at opposite ends of a cylindrical housing, which housing provides an hermetic enclosure including an ionizable gas such as argon. A narrow gap is formed between the electrodes so that normally there is no conduction through the device. However, when a surge of sufficient magnitude and duration appears at the electrodes, the gas in the gap is sufficiently ionized so that the gap breaks down and current is conducted through the device and away from the protected apparatus which is connected in parallel therewith. (For an example of a narrow gap surge limiter, see, for example, U.S. Pat. No. 4,175,277 issued to Zuk.)
In many electrical systems, a pair of lines is utilized to carry the current signal so that the lines are usually at an approximately equal voltage with respect to ground. In such systems, a dual-gap surge limiter is typically employed. This device includes a third electrode between the previously described end electrodes so that separate gaps are formed between the third electrode and the end electrodes. The end electrodes are coupled to the signal lines and the middle electrode is coupled to ground. (See, e.g., U.S. Pat. No. 3,934,175 issued to Clark.)
In the event of a surge, it is desirable to have both gaps break down essentially simultaneously so that the surge will be shunted from both lines to ground and balanced operation is achieved. However, because of voltage division effects on the lines, the voltage across the gap which has not broken down is reduced immediately upon the breakdown of the first gap. Thus, the breakdown of the second gap must be induced to occur at a voltage which is lower than the normal breakdown voltage for the device in order to achieve balanced operation. As far as applicants are aware, it has not been possible prior to this invention to have the second gap break down reliably and independently of polarity when the voltage across the second gap is substantially reduced (typically to less than 20% of the device breakdown voltage).
Thus, reliance was typically placed on providing elements external to the surge limiter for achieving this balance. (See, e.g., U.S. patent application of S. Hong, Ser. No. 493,997, filed May 12, 1983 and assigned to the present assignee.)
It is, therefore, a primary object of the invention to provide a protector with at least two gaps where the gaps will break down essentially simultaneously when a surge of either polarity appears on the device electrodes even when the voltage across the second gap is reduced to less than 20% of the device breakdown voltage.